Gauge for measuring the conductance of a liquid present between two electrodes

ABSTRACT

A liquid level gauge consists of two main cells comprising a measuring electrode, a compensation electrode and their respective counter electrodes. Circuitry is provided to determine the conductance of the liquid in each cell. Further circuitry is provided to calculate the quotient of the respective cell conductances, which is indicative of the liquid level. The surface areas of the measuring and compensating electrodes are kept equal, but the distance between the measuring electrode and its counter electrode increases linearly, while the distance between the compensation electrode and its counter electrode is constant.

CROSS-REFERENCE TO PARENT APPLICATION

This is a division of application Ser. No. 677,506 filed 3rd Dec. 1984.

The invention relates to a level gauge of the conductive type formeasuring the liquid level in a vessel or flow-tube in a conductive way,whereto a measuring electrode (M) and a reference electrode (R) areprovided in the vessel or the like, which each cooperate with a wall orpart thereof, acting then as counter-electrode to form conductance cells(G_(M) and G_(R)) with the liquid to be measured in the vessel or thelike acting as conductive medium, in which the quotient (G_(M) /G_(R))is directly proportional to the liquid height.

Level gauges are being applied in liquid vessels, tanks (such as oiltanks of petrol service stations) and the like containers, in which thegauge does not only measures the liquid level, but also should be ableto signalize any deviation from the composition of the liquid. Aparttherefrom such gauges should be able to be used for controlling waterlevels in rivers and the like.

Gauges for determining the liquid height being closest related to thegauge of this invention are of the capacitive type: the level gauges ofthat type could be divided into three categories:

I. The capacitive level gauge with one single electrode (standardgauge).

II. The capacitive level gauge with reference electrode (referencegauge)

III. Three terminal method with the auxiliary electrode divided intoparts.

I. The gauge of the first category.

A rod-like measuring electrode is placed in the centre of the vessel orthe like, because it cooperates with the wall of the vessel acting ascounter-electrode. The capacitor so formed, is of the cylindrical typeand its capacity is defined by the formula ##EQU1## in which

ε_(r) =the dielectric constant of the liquid

L=the length with which the rod-electrode reaches into the liquid

D=the distance of the rod-electrode to the wall

d=the diameter of the rod-electrode

The measured capacity is proportional with L and hence a measure for theliquid height. In view of the big distance from the centre of the vesselto the wall of the vessel, the sensitivity of the gauge is not great.Another disadvantage is that the electrode does not extend to the bottomof the vessel and therefore the L is not quite the same as the liquidheight H. There are still a number of other factors which affect themeasurement, so that the eventual precision to be obtained is not betterthan ±2%. In many cases, however, this is amply sufficient.

Besides by placing the measuring electrode within a measuring tube, thesensitivity of the measurement is greatly enhanced. This measuring tubeserves then as counter-electrode of the capacitor.

II. The gauge of the second category.

In the extension of the central measurement electrode (M) at thelower-side thereof, is place a reference electrode (R), which electrodeseach form with the wall of the vessel or of the measuring tube acapacity C_(M) or C_(R). The quotient of these capacities yields asignal being proportional to the liquid height because the capacity ofthe reference electrode, once the liquid in the vessel or the like hasrisen past the reference electrode is constant. However, if thecomposition of the liquid (for example oil) in the vicinity of thereference electrode deviates from that of the liquid in the vicinity ofthe measuring electrode, it does have influence on the measurement ofthe quotient signal. However, it is not simple to take into account acorrection for this type of errors. Especially if water collects nearthe reference electrode, then as a consequence of the fact that thedielectric constant of water is 80 times that of air, the measurementevidently becomes disturbed to a large extent without it being knownexactly how to correct this fault.

III. Three terminal method.

Here the auxiliary electrode is constructed from a number ofsuperimposed sub-electrodes being spaced mutually a certain distance. Ifa sub-electrode, somewhere on the electrode, is partly covered with aliquid, said part can be measured between 0 and 100% of the scale. Thesub-electrode, preceding the sub-electrode in consideration, iscompletely immersed into the liquid and serves as reference for changesin composition of the liquid. Still there is a portion above themeasuring portion, which is completely in the gas phase; that upperportion serves as reference for changes in the gas-cap.

All covered electrode parts are processed in a memory so that the levelcan be measured rather precisely. The manner in which "references" aremade, makes the gauge suitable for small level ranges, viz.corresponding to the height of each sub-electrode, which, by means of amemory, are summed up tot the total height.

IV. Capacitive level gauge with automatic compensation.

For the reasons aforementioned the reference-electrode (R) of the levelgauge according to the invention has been replaced by a compensationelectrode (C). This liquid gauge distinguishes itself therefore by thefact that the reference electrode is constructed as a compensationelectrode, being of the same length as the measuring electrode andarranged parallel thereto, in which the difference in shape or surfacearea or distance with respect to the counter-electrode between themeasuring and compensation electrode-pairs is such that the result ofthe division of the capacities (C_(M) and C_(R)) indicates a linearrelationship of the liquid height or a relationship of higher order.

The parent application is concerned with capacitive measurements betweenpairs of electrodes, arranged in a liquid, in which either thedifference in electrode surface areas or the difference in electrodedistances function as a liquid height dependent variable.

Under circumstances it can be necessary to measure the conductance of aliquid. The striking point is that the variables being of interest formeasuring the height, viz. the surfaces shape of or the distance betweenthe electrodes, are the same both in the capacitive and in theconductive measurement. Depending on the kind of liquid it may bepreferred in one case to do the measurement in a capacitive way, inanother case in a conductive way, whereas, if required, at the same timethe dielectric constant or the specific conductance of the liquid can bemeasured.

For further details of the capacitive measurement, reference is made tothe parent application, since this present application is furtherrestricted to the conductive aspects of the invention.

It will be clear that for the measurement of the conductance of a liquidin principle the same electrode arrangements can be used. Only theelectrodes must be bare (i.e. not-coated to protect againstshort-circuiting what is not necessary here) and the electronicstherefore should be adapted. The results are then quite the same as withthe capacitive measurements. Also here changes inthe conductance (G) ofthe liquids can be compensated automatically with the models hereafterto be discussed, or even--in addition to the height measurement--theconductivity (χ) itself can be determined.

The invention is therefore also related to a guage for measuring theconductivity (χ) of a liquid, present between two opposite electrodes ina vessel or conduit-pipe, to which end a measuring electrode (M) andreference electrode (R) are provided in the vessel or the like,cooperating with a wall or part thereof, acting as counter-electrode, soas to form cells having a conductance G_(M) or G_(R) with the measuringliquid in the vessel or the like between said electrodes as conductingmedium, in which the quotient of G_(M) and G_(R) is a measure for theheight or level of the liquid. This gauge is characterized in that thereference-electrode is formed as a compensation electrode being of thesame length as the measuring electrode and arranged parallel thereto, inwhich the differences in shape or surface area or in distance withrespect to the counter-electrode between the measuring and compensationelectrodes is such that the results of the dividing operation of theconductances G_(M) and G_(R) indicates a linear relationship with theliquid height.

Owing to this embodiment the compensation electrode "sees" at each levelthe same liquid compensation "seen" by the measuring electrode. Thusthere cannot longer occur differences in the liquid composition whichsaid one electrode does "see" and said other electrode does not "see".In this manner a kind of automatic compensation is obtained. Thisautomatic compensation is due to the fact that the compensationelectrode-contrary to the prior reference electrode-is of the samelength as the measuring electrode. The associated electronics can becompared with the electronics of the reference-system, but it has morepossibilities.

In a practical embodiment the compensation electrode is placed next tothe measuring electrode and the two electrodes are provided on one andthe same rod ("sandwich" type). The compensation electrode is in thesimplest form a straight dimensioned element, provided on a plasticssubstrate.

On the other side is provided the measuring electrode. This entity,constructed as sandwich, is put in a square measuring tube forming thecounter electrode both for the compensation electrode and the measuringelectrode. Thus the formula for the conductance G_(M) of the measuringcell will be: ##EQU2##

Since the compensation electrode is straight and is disposed parallel toa pair of opposing straight walls of the square measuring tube, aconductance G_(C) is measured therebetween, being variable with thesurface area S or the distance l, as is also the case in formula (2a) towit ##EQU3## in which:

χ=conductivity of specific conductance of the liquid

S=surface area of the immersed part of the electrode

l=distance between main electrode and its counter-electrode (on themeasuring tube)

The invention is based on the recognition that there should be adifference in either shape or in distance between the two mainelectrodes of the two cells, because if they are the same, thedivision-result is no function of the liquid height.

So there are two principles of conductance measurement (like in theparent case of capacitance measurement):

A. the distance l is kept constat, but the surface shape or area S₁ ofthe measuring electrode is variable. The ratio of the conductances ofthe two cells becomes: ##EQU4## Preferably l₁ =l₂, then ##EQU5## SinceS=h.b, in which

b=width of the electrode plates, we find ##EQU6## b₁ =constant and b₂=f(h)

Thus G_(M) /G_(C) =f(h)

B. the surface shape or area (S₁, S₂) is kept constant, but the distancel₂ is variable, whereas l₁ =constant. ##EQU7## Preferably S₁ =S₂. We get##EQU8##

For this reason the level gauge of the invention is in the firstinstance (principle A) characterized in that the compensation electrodehas the shape of a rectangle and the measuring electrode that of arighttangled triangle. These "mathematical" shapes are applicable bothon flat plates and on cylindrical plates. In this manner a continuousreading of the liquid height is obtained which appeared to be notpossible thusfar with the existing liquid level gauges.

In principle the deviation in shape between the measuring electrode andthe compensation electrode can be at random, provided the result of thedivision of the two conductances do not yield a constant, but acontinuously varuing value with increasing height. By the above measurethis leads to a linear relation, although another continuous relation isalso useful.

The same linear relation is obtained, if the compensation electrode hasthe shape of a rightangled triangle and the measuring electrode that ofa parabola.

Under certain circumstances it is of advantage when the slanting side ofthe triangle has a stepwise profile.

The new system lends itself for measuring both great differences inheight over tens of meters, as in large storage tanks, and smalldifferences in height of some centimeters. In the latter case, and ifthe horizontal dimension of the liquid basin permits same, a gauge is tobe preferred, having in vertical direction smaller and in a horizontaldirection greater dimensions. Then a gauge is obtained, characterized inthat the mathematical relationship expressed by the shape of thecompensation and measuring electrodes respectively, includes for each ofthem also a relationship of higher order, provided there be alwaysbetween them a difference in order equal to one.

Now too the result is linear. If another than linear relation isdesired, for example a square one, a gauge is recommended, characterizedin that the shape of the compensation and measuring electrodes is suchthat the result of the division of the two conductances (G_(M) andG_(R)) indicates a relationship of second order. Preferably themeasuring and compensation electrodes are provided on either side of oneand the same insulating substrate or carrier.

Other applications of the present conductive type level gauge relate to:

A. Interface measurement.

This subject includes the following sub-subjects:

1. The precision and sensitivity resp. of the measurement with a"normal" oil-water mixture;

2. Use of the triangle with a "reversed" two liquid layer system;

3. Use of the triangle with a three liquid layer system.

B. Measurement based on distance-variation

A. Interface measurement.

1. Sensitivity of the measurement.

The liquid gauge is suitable virtually for any liquid. But a particularapplication terrain is reserved for the present gauge when measuring theliquid level of hydrocarbons (oil or petrol) in storage tanks. Whenthese liquids have just left the source, they contain a given percentageof water. In formula (2) for the conductance, being a measure for theliquid level in the reservoir, χ plays an important role. Organicsubstances, like oil, have a high resistivity in the order of MΩ, thatis a very low conductivity χ, but water, being present in the oil insmall quantities, has a low resistivity in the order of kΩ, that means arelative high conductivity ω. Or, if the water were homogeneouslydivided within the oil, then there were no problems, for, the average χwould be only a fraction greater than the χ of oil. The problems arisehowever, since the water is not dissolved into the oil, but irregularlydistributed within the oil.

If the sensitivity of the gauge is set for oil, because this is here theuseful liquid, whereas the water is the undesired component of themixture, and if all at a sudden water appears in the region thatfunctions as medium for the conductive cell, then the gauge will not beable to work, will get overranged and will not be able to accomplish anymeasurement of the oil liquid level, as long as that water is in theneighbourhood.

One has tried to neutralize this disturbing presence of water byhomogenising the liquid flowing along the gauge, containing one momentwater, the next moment no water, by means of a stirring apparatus, butthis method is unpractical, since one prefers to keep oil and waterapart.

By means of the gauge according to the invention, it is possible, inspite of the presence of water in the crude oil, to measure the liquidlevel with reasonable precision over the entire height of the "vessel"without homogenisation.

An example as illustration:

(a) of the existing situation:

The signals E_(M) (measuring conductive cell) and E_(R) (compensatingconductive cell) go to amplifiers being adjusted for measuringhydrocarbons adjusted at an average χ in connection with possible smallvariations in χ for oil in the dry state.

If there is a water layer under the hydrocarbon level, then theamplifier will quickly be driven to its full power (into saturation)because χ_(H).sbsb.2_(O) =±10³ ·χ_(oil) and therefore this variable is amultiplying factor in the conductance-formula, so that further risingsof the liquid level are not "noticed" by the gauge.

(b) of the improved situation:

The amplifier is driven (within its linear control-range) to for example50% of the scale-falue for a full tank, filled with an organic liquid,for which χ=low so that for liquids having a χ that varies between asmall, fixed range, the amplifier will be driven up to its full power,that means: is controllable over its total linear control range from 0to 100%. The remainder (second half) of the scale range can be employedfor variations in χ (past said range) and for further risings of theliquid level.

2. Influence water content on the measurement.

As long as the oil is "dry", the amplifier-on the base of theeffectuated adjustments-can never be controlled or driven to beyond itslinear control range (get over-ranged). In normal cases the levelindication is situated in a tank full of oil and with a χ-value at 50%of the scale. If this oil has a 20% greater χ-value, then at 60% of thescale. Just because of fluctuations in the χ of oil, the provision ismade to have a little clearance in the scale deflection in reserve. If,however, the oil is mixed with small quantities of water, thus when theoil is "wet", the output of the amplifier will increase to beyond 100%(beyond its linear range); the amplifier now becomes over-ranged (out ofrange). A circuitry provides for setting at that 100% a new "zero"conductance (G_(M)) but now at a higher measuring range, such that theamplifier is again driven to 50% (or half) of its linear controllingcapacity for a χ variation in a next small range. This switching-over isindicated visually by the glowing-up of a LED.

If the amplifier exceeds again its maximum (100%) linear controllingcapacity (due to a new layer of water, or still due to the "old" layer)switching-over to higher scale-ranges will continue until at last arange is found, in which the amplifier can operate between 50 and 100%of its controlling capacity.

Each time that the thickness of the water layer occurring in the oilbetween the electrodes, exceeds a given threshold value, in which theoutput of the amplifier is driven up to full power (more than 100%) andthus the amplifier becomes "over-ranged", this switching-over towards ahigher measuring range takes place, for example for every 2.5 cmthickness of the water layer. The number of glowing LED's indicates thenhow often said switching-over has occurred and how thick the total waterlayer between the electrodes is.

To this end the conductive type level gauge according to the inventionis characterized in that for measuring the interface between anunderlying heavier liquid and a superimposing useful liquid, thetriangle electrode is placed into the liquid having its narrow sideturned downwards.

Thus finally in the result of the measured liquid height twoinexactitudes can be present, which are caused by the fact that

(a) the oil liquid, for which one has started from a low χ-value,comprises a component with a 20% higher χ-value;

(b) the water as a whole has not yet reached the layer thickness atwhich switching-over to a higher measuring range occurs. This is alsovalid, when switching-over has already taken place one or more times,for the still remaining H₂ O, if any.

These deviations, however, remain completely within the precisionconditions which this kind of gauges is subjected to by the Inspectionof Weights and Measures. The imperfections are therefore virtuallynegligible, so that it can be stated rightfully that thanks to themeasures according to the invention, viz.

using the driving or controlling capacity of the amplifier only for 50%of its linear range;

switching-over to a higher measuring range, and

signalling each of these transitions for example by means of aLED-indication (or a counter), the level measurement itself is notaffected essentially by the presence of these "disturbing" components.

If the measuring amplifier switches-over, the reference amplifier willchange too, proportionally, so that the ratio between the twoconductances (G_(M) and G_(R)) remains the same. In this way too thelevel measurement has not changed.

The numerical water thickness indication by LED's (or counter) can bereplaced by an analogous indication.

3. Use of the triangle with a "reversed liquid system".

(a) In the preceding part of the speficition there has always beenquestion of a straight electrode for the reference signal and atriangular electrode (called "triangle") having its narrower part turneddownwards for the measuring signal, when there is a question of a"normal" oil-water-system, in which water is in the minority. In thatcase oil is the useful liquid which is to be measured. Thanks to theaforementioned measure the water is, as it were, becoming eliminated, sothat always a half scale-range is left for controlling the useful liquid(oil) in the event that oil with a higher χ value is present in theliquid. In that case the triangle is disposed "correctly". In order tojudge whether the triangle is disposed "correctly" or "wrong", oneshould watch the interface line, that means the bordering line betweentwo liquids. In the "normal" case the underlying liquid is for examplewater (χ=high) and the superimposed, lighter in weight, liquid is forexample oil (χ=low).

If the vessel is (almost) full with oil, the interface line is virtuallynear the bottom. The ratio of the surfaces wetted by water is then S₁:S₂ =1:11/2.

If the vessel is (almost) completely filled with water, the interfaceline is (nearly) at the top. The ratio of the surfaces wetted by wateris now S₁ :S₂ =1:51/2.

Between these ratio-limits (minimum=1 and maximum=6 when the slantingside of the triangly forms an angle of 45° with the vertical) it shouldbe possible to measure the interface. It is clear that interfacemeasuring requires quite another adjustment of the control capacity ofthe amplifier, whereas besides always should be taken into accountfluctuations in the χ of the oil.

(b) In the "reversed" case ther is in the lower-part of the tank a heavyoil layer, for example "ECH" (ethylchloroheptane, specific density γ=4to 5). On this liquid the water, being lighter in weight, floats.

If the vessel is (almost) completely filled with the heavy oil, theinterface is (nearly) at the top, and the ratio between the surfaceareas is S₁ :S₂ =1:101/2.

In the event that the vessel is (almost) completely filled with water,the interface is (nearly) at the bottom and the surface area ratio is S₁:S₂ =1.61/2.

It will be clear that in such a case the gauge operates "reversely". Thegauge measures with increasing quantity of water a diminishing surfacearea ratio, viz from 1:11 (maximum) to 1:6 (minimum). On the scale itwould become necessary to read from 100% to 0%. In order to exclude this"illogical" situation, the triangle is, each time that the "interface"measurement occurs in a "reversed" liquid system, reversed too. Thuswhen the vessel is (almost) completely filled with oil, the interface is(almost) at the top, and for the surface area ratio is valid: S₁ :S₂=1:11/2.

In the case that the vessel is (almost) completely filled with water,the interface is (nearly) at the bottom, and the surface ratio withincreasing water quantity is 1:51/2. By this reversal of the triangle itis possible with increasing quantity of water in the vessel, to measurebetween surface ratios from 1-11. Remark: Heretofore it has beenexplained that on applying the gauge in a "reversed" liquid system thetriangled measuring electrode must be reversed. However, when in thedividing operation the components of the quotient are reversed, thusG_(R) /G_(M) is determined, the same result is obtained, however,without reversal of the measuring electrode.

4. Three liquid layer system.

If the composition of the liquid in the vessel is such that the heavyoil is in the lower part of the vessel, water floating there-above and alight weight oil layer floating on said water, then the two interfacescan be monitored by means of a double electrode system of compensationand measuring electrodes, in which the triangles of both systems aredisposed opposite each other in mirror image reflection with respect toa horizontal plane and are facing each other with their narrow sides.The interface of the upper two-liquid system is measured by the upperelectrode system, and the lower interface of the lower two-liquid systemby the lower electrode system.

However, it will be clear that depending on the specific weight of thecomponents in a combination of three liquids, also those cases canoccur, in which the two triangles are disposed each with their narrowside turned downwards, or turned upwards, or are facing each other withtheir wide side in mirror image reflection.

This system can be extended, without any restriction, to any number ofinterfaces, in which the position of the triangles opposite each otheris defined by the specific weight of the liquids, which meet each otherat the interface.

B. Measurements based on distance-variation.

In the gauge thusfar described the main electrodes of the measuring celland the compensating cell are plan-parallel to their counter-elecrrode.The counter-electrodes of the two cells are part of the surrounding wallof the measuring tube. The measuring tube can be considered as a rightangled triangle and the compensation electrode as a straight narrowstrip. A level gauge provided with such a measuring electrode can beused for level measurements with great precision.

The invention has moreover for its object to propose other combinationsof measuring electrode and measuring tube, which are based on the sameprinciple and with which the same, great precision can be obtained.Hence, according to an alternative embodiment of this inventiveprinciple the measuring electrode (M) and the wall part of the measuringtube acting as counter-electrode, are so disposed opposite each other,that the distance inbetween, in horizontal direction, isvariable--whereas the distance between the compensation electrode (R)and the opposite wall part of the measuring tube is constant--so thatdivision of the conductances (G_(M) and G_(R)) yields a linear or higherorder relationship with the liquid height to be measured.

In the previous embodiment the distance from the measuring andcompensation electrodes to the associated wall of the measuring tuberemains constant; solely the surface of the measuring electrodeincreases with the second power. Now in the alternative embodiment ofthe gauge according to the invention, said surface remains constant, butthe distance varies.

In a first embodiment of this alternative idea, this leads to themeasure, that the wall part of the measuring tube opposite the measuringelectrode (M) and/or the measuring electrode itself is slanting. Thecompensation electrode and cooperating wall of the measuring tube are inprinciple straight. They can also be slanting, provided they remainparallel to each other.

This can anyway be realized in two different manners. In the first placesuch that for use in a "normal" liquid (for example oil-water) systemthe wall part of the measuring tube cooperating with the measuringelectrode is inclined, its bottom edge being nearer to the axis of thegauge than its top edge. Under a "normal" two-liquid layer system, hereoil-water, is to be understood a system in which the useful liquid (oil)floats on a layer of the polluting liquid (water).

In the second place this realisation is possible, in that for use in a"normal" liquid system the wall part of the measuring tube cooperatingwith the measuring electrode is straight, while the measuring electrodeitself is inclined, its top edge being nearer to the axis than itsbottom edge. In these cases measurement of the reversed quotient ofG_(M) and G_(R), so G_(R) /G_(M) leads to a rising linear relation withthe height of the liquid level.

In the case of a "reversed" liquid-system one has to see to it, thatwhen one likes to measure the reversed quotient Q'=G_(R) /G_(M), thegreatest distance between the measuring electrode and cooperating wallof the measuring tube is at the lower end and the smallest distancetherebetween is at the upper end. Remark: It will, however, be clearthat for obtaining a directly proportional relation between the quotientand the liquid level, one can also take the quotient Q=G_(M) /G_(R) (asin the previous embodiment) and that when applying the gauge in a"normal" liquid system, provision has to be made that at the upper endthe distance-between measuring electrode and cooperating wall of themeasuring tube, is smaller than at the lower end, but when applying thegauge in a "reversed" liquid system, provision must be made that thedistance at the upper end is greater than at the lower end.

For convenience's sake an electrode carrier body is applied, on whichthe measuring and compensation electrodes have been applied. A preferredembodiment is so constructed that in a symmetrical arrangement of amulti-sided carrier-body for the electrodes, the wall parts of themeasuring tube on the one sid and on the other side, cooperate with themeasuring electrodes on the one side and on the other side, and/or saidmeasuring electrodes themselves are inclined, whereas the compensationelectrodes are provided on the carrier-body opposite each other on thetwo other sides and have a constant distance with respect to the opposedwall parts of the measuring tube.

Another embodiment of the carrier body is constructed such that thecarrier-body has in cross-section the shape of a triangle, and in thatthe measuring electrodes are provided on two sides and the compensationelectrode on the third side. Of course, the carrier-body can also beformed as a body of revolution (cone, cylinder and the like).

These gauges can also be applied in multi-liquid layer systems with twoor more "interfaces". In that case one puts on or below the measuringtube section, having an inclined wall part, whose lower end is nearer tothe axis thas its upper end, another section having likewise an inclinedwall part, but whose lower end is farther from the axis than its upperend. The total wall of the combined measuring tube sections or measuringelectrodes exhibits a kink, viz. an outwardly directed (or "convex")kink or an inwardly directed (or "concave") kink. Of course more thanone of these kinks can be applied, also combinations thereof. Hence, itis recommended to apply in such situations a measuring tube,characterized in that the wall part cooperating with an electrode and/orthe electrode itself exhibit one or more kinks.

Since over great heights of the liquid level gauge the inclination ofthe wall or of the measuring electrode can lead to an unacceptableextent on the gauge, measures are proposed to meet this inconvenience,viz. in that the wall part cooperating with the measuring electrode isformed with a stepwise profile and the measuring electrode itself isdivided correspondingly into mutually separated sub-electrodes. All thisboils down to the fact that the level gauge is divided into a number ofindividual level gauges, in which the inclined wall of the tube sectioneach time jumps back (is off-set). In the measuring circuitry propereach time a switching-over to a higher measuring range occurs.

The invention need not relate exclusively to level measuring apparatus.It is quite well conceivable that the construction shapes are alsoapplicable to other measuring situations, for example: in through-flowinstallations, pressure measurements, weighing technics, wind tunnel,etc.

The invention is hereinafter described with reference to theaccompanying drawings, in which

FIG. 1-6 show various level gauges of the capacitive type of the priorart, and FIG. 7-33 pertain to a level gauge of the conductive typeaccording to the invention, viz:

FIG. 1 shows a level gauge of the prior art, provided with oneelectrode;

FIG. 2 shows the electric circuitry thereof;

FIG. 3 shows a measuring arrangement of the prior art of the type inFIG. 1, placed within a metal measuring pipe;

FIG. 4 shows a second type of level gauge, also of the prior art, butwith a second electrode as reference electrode;

FIG. 5 shows the electric circuitry thereof;

FIG. 6 shows a third type of level gauge, again of the prior art, butwith an electrode divided into sub-electrodes;

FIG. 7 shows a level gauge of the conductive cell type with compensationelectrode;

FIG. 8 shows a structure thereof in a measuring pipe;

FIG. 9 indicates the relation between thee height of the liquid and themeasured conductance;

FIG. 10 shows a level gauge with compensation electrode according to theinvention;

FIG. 11 illustrates the relation between liquid height and the measuredconductance, analogous to FIG. 9;

FIG. 12 is similar to FIG. 11, but indicates the measuring points, so asthey really have been found;

FIG. 13 shows an alternative of the embodiment of FIG. 10;

FIG. 14A, B show an electrode arrangement of a level gauge according tothe invention with a "normal" two-liquid layer system;

FIG. 15A, B shows the same electrode arrangement of FIG. 14, applied toa "reversed" two-liquid layer system;

FIG. 16A, B show the same as in FIG. 15, but now the electrodearrangement has also been reversed;

FIGS. 17-20 show four possible electrode arrangements, to be used withvarious three-liquid layer systems;

FIG. 21 shows again a triangular measuring electrode, in which theinfluence is investigated of modifying the position of the slantingside;

FIG. 21A shows a modification of the triangled electrode, such as forexample illustrated in FIG. 10;

FIG. 22A-C shows a first embodiment of the invention, in which one wallof the measuring tube is inclined, the lower part of said one wall beingcloser to the measuring electrode than its upper part;

FIG. 23A-C illustrate an alternative embodiment of FIG. 22A-C, in whichthe measuring electrode is inclined, its lower part being closer to theassociated wall of the measuring tube than its upper part;

FIG. 24A-C shows another alternative of FIG. 22A-C for a "reversed"liquid system;

FIG. 25A-F are illustrative for the term "reversed liquid system";

FIG. 26A-D and FIG. 27A-D show a second embodiment, in which the carrierof the electrodes has four sidewalls of which two opposed walls areinclined;

FIG. 26A-D for a normal liquid system;

FIG. 27A-D for a "reversed" liquid system;

FIG. 28A and B show an alternative embodiment of FIG. 26, in which theelectrode carrier is in cross-section triangular;

FIG. 28C shows an embodiment in which the measuring tube or electrode isconical;

FIG. 29A-E show an embodiment, in which two or more measuring tubesections, as represented in FIG. 22A-24C, are combined together, so thatthe combination of measuring tube sections and/or electrodes exhibits onthe transition place between the sections, a kink; and

FIG. 30A and B show an embodiment in which the sections of the measuringtube connect to each other stepwise.

In FIG. 1 is shown a level gauge of the standard type, being placed in aliquid vessel 2. The gauge is inserted into the cylindrical vessel 2 bymeans of a centrally arranged cylindrical pin 3 covered with electrodematerial or any other cylindrical object. The wall 4 of said vesselfunctions as counter-electrode. With the liquid 5 as dielectric betweenthe electrodes 3, 4 the determination of the liquid height in the vesselis reduced to a capacity measurement, when between the cylinder-plateshaped electrodes an alternating voltage of voltage source 7 isconnected.

The capacity of the capacitor thus formed is proportional with theheight, over which the measuring electrode 3 is inserted in the liquid5. The electric circuitry is shown in FIG. 2. The signal i obtained fromthe liquid measurement can be transformed directly into aheight-reading, expressed in cm or m, if may be desired.

Since the radius R appears inthe denominator of the capacitor formula,the capacity of the capacitor formed is small and therefore also thesensitivity.

By putting the measuring electrode into a pipe or tube, this objectioncan be met with. In FIG. 3 the measuring electrode 3 is disposedcentrally within a measuring pipe 8, functioning as counter-electrode insubstitution or the vessel wall 2. Inhomogenities in the liquid cannotbe measured by this gauge. Also the measured height does not quitecorrespond with the real height. One can provide therefore a fixedcorrection, so that still the correct value can directly be read.

In FIG. 4 and 5 is shown a second type of a known capacitor level gauge,in which a reference electrode 11 is applied. This reference electrodeallowed to eliminate the influence, which changes in the dielectricconstant of the liquid can have on the measurement, within given limitsand provided the liquid be homogeneous.

If the liquid is not homogeneous in composition, greater deviations withthe reference electrode can arise than with a normal standardmeasurement. As shown in FIG. 4 the measuring electrode 3 is on itslower side provided with a reference electrode 11. The whole thing isput into a steel measuring pipe 8. When the reference electrode iscompletely covered, the signal issued will only change by a modificationof the dielectric constant ε_(rx) of the liquid to be measured.

The measuring signal of the reference electrode is passed to theamplifier 17 (FIG. 5) via oscillator 14. The measuring signal of themeasuring electrode is passed to amplifier 16 via oscillator 13. Bothsignals are passed to an amplifier 18, which conducts the division suchthat only the level height is shown. If now the dielectric constant ofthe liquid changes, this will have no effect on the reading, that means:will not be observed by the gauge.

The above statement only holds of the dielectric constant changes overthe entire measuring region. Further the change in dielectric constantmay not be greater than ±10% of the original value. When the greaterchanges occur, the amplifier cannot handle this sufficiently so thatdeviations will occur.

This disadvantages of the reference measuring system are that not alleffects which can disturb the measurement, are eliminated, such as amongothers the fact that the dielectric constant of the liquid near thebottom of the vessel-thus at the reference electrode-can be considerablygreater than at the measuring electrode. If the reference electrode hassome pollution, this can influence the reference signal. Because thereference electrode is placed down in the vessel, the change ofpollution is there greatest. The gauge can only function well if thereference electrode is completely covered with liquid. The lower part inthe vessel can therefore not be measured.

As appears from the above remarks, the reference measurement is notalways reliable and only very restrictively usable.

In FIG. 6 a third type of known capacitive level gauge is shown, inwhich next to the measuring electrode a split reference electrode 21 ispresent. By the division of the reference electrode 21 into a pluralityof small sub-electrodes 21A, inhomogenities in the dielectric can bedetermined more exactly, so that a similar device for localising theintermediary plane or interface 22 between two liquids 23 and 24, andthe interface 26 between the liquid 24 and the gas cap 27 is suitable.

The disadvantages of the already previously discussed referencemeasuring system can be removed with the automatic compensation system,FIGS. 7-9. In the compensation system on each level of the measuringelectrode 3 a feed back is given, originating from a compensationelectrode 31. The compensation electrode 31 is put next to the measuringelectrode 3 and mounted on the same carrier rod 32. The two-electrodesare of the same length. The associated electronics can be compared withthe electronics of the reference system (FIG. 5) but has morepossibilities, as will be discussed later on. The compensation electrode31 is in principle a straight dimensioned element, provided on aplastics substrate 32 (FIG. 7). The electrode is placed into for examplea square measuring pipe 8, forming one plate of the conductive typeplate cell, such that d₁ =d₂. The compensation electrode 31 measureswith respect to the measuring pipe 8.

The signal issued by the compensation electrode 31 is shown in FIG. 9;one obtains a linear relationship. The same applies to the measuringelectrode itself, so that on dividing the signals, the quotient is a"constant" and the system as such is unsuitable for automaticcompensation.

In FIG. 10 is shown a conductive measuring system according to theinvention. The measuring electrode 33 is in principle disposed next tothe compensation electrode 31 and forms a measuring cell with themeasuring pipe 8 (not-shown), of which the wall acting ascounter-electrode, is spaced a distance l from and parallel to theelectrodes 31 and 33. The fact that the surface S₂ of the measuringelectrode 33 is divided by the surfaces S₁ of the compensation electrode31 explains why the measuring electrode 33 in the upward direction mustlinearly increase in surface area, in order to obtain a measuring signalthat is directly proportional with the liquid height. As can be seeneasily, in the system of FIG. 10 holds the quotient Q ##EQU9## in whichA_(i) en B_(i) are the sub-surfaces of the measuring electrode 33 andthe compensation electrode respectively. Elaboration of the formulagives: ##EQU10##

X=conductivity associated with the liquid level.

From the preceding formula it appears that the output signal of thedifferential amplifier is directly proportional with the liquid height,independent of the course of the specific conductance of the liquid andthe gas cap.

Both signals M and R can be represented in one figure, inclusive thecompensated output signal U (FIG. 11). The disadvantages adhering to thetwo preceding measuring systems, are eliminated with the compensationsystem. With said compensation system is obtained a purily conductivemeasuring signal that does not deviate from the real liquid height.

FIG. 12 is identical to FIG. 11, but on a larger scale, in which theexperimental measuring points are represented exactly.

In FIG. 13 is shown a second embodiment of the measuring systemaccording to the invention, with compensation electrode. In thisembodiment the compensation electrode 36 is formed as a triangle,whereas the measuring electrode 38 has one of its sides formed as aparabola 39.

In FIG. 14A, B is shown an electrode-arrangment consisting of arectangular compensation electrode 41 and a triangular measuringelectrode 42. This arrangement is the same as that of FIG. 10. Theelectrodes are placed into a "normal" two-liquid layer system, forexample oil and water, of which system the interface line is shown asdotted line 43. In FIG. 14A it is supposed that the vessel (not shown)is almost completely filled with oil. The interface line 43 is therefore"down". The ratio of the water wetted surfaces of the two electrodes 41,42, so that the quotient Q or the ratios S₂ /S₁ is--as can easily becalculated--equal to 11/2. In FIG. 14B is assumed that the vessel is"almost" full with water. The interface 43 is therefore "up" and theratio of the surfaces is 51/2. In the extreme case there must be acontrolling range between the minimum value (=1) of the quotient and itsmaximum value (=6).

In FIG. 15A, B the electrodes 41, 42 are placed into a "reversed"two-liquid layer system, that is a system in which the usefull component(oil) is not above the useless component (water), but below it. In FIG.15A it is supposed that the vessel is (almost) completely filled withECH and thus the dotted line of the interface 44 is drawn "up". Thatposition corresponds with a surface ratio of water wetted surfaces of101/2. In FIG. 15B it is supposed that the vessel is (almost) completelyfilled with water. The interface 44 is "down" and yields a Q=61/2. Inthis case with increasing quantity of water in the vessel, the quotientQ will decrease; in the extreme case from 11 to 6. This is illogical.

Hence in FIG. 16A, B the triangular electrode 46 has been turned upsidedown. In FIG. 16A the vallue of Q is now 11/2 when there is little waterin the vessel and 51/2 when there is much water in the vessel. In sodoing the gauge of the "reversed" liquid system operates similarly as inFIG. 14 for the "normal" liquid system, viz. from 1 to 6.

In FIGS. 17-20 are shown electrode arrangements 51, 52 which can beemployed, if in a vessel, tank or storage container a three-liquid layersystem is present, with two interfaces 53, 54.

In FIG. 17 the case is shown that the upper-two liquid system(oil-water) is a "normal" system, so that the triangle 56 of themeasuring electrode has its narrow side 57 turned downwards. The lowertwo-liquid system (water-ECH) is however a "reversed" system andtherefore the triangular electrode 58 has also been reversed and itsnarrow side 59 is turned upwards. One can also say that the twoelectrode arrangements have been mirror-image reflected about thesymmetry-axis 60.

In FIG. 18 the two-electrode arrangements 61, 62 have also beenmirror-image reflected mutually, but now the normal electrodearrangement 62 is "down" and the "reversed" arrangement 61 is "up".

In FIG. 19 and 20 there are still shown two other electrode arrangements71, 72; 76, 77, in which in FIG. 19 there is question of two normaltwo-liquid layer systems, one atop the other, and in FIG. 20 of two"reversed" two-liquid systems, one atop the other.

The triangular electrode 81 drawn in the preceding figures, has alwaysbeen shown as an isosceles rightangled triangle, thus in which theslanting line forms an angle α=45° with the vertical. In that case thequotient Q varies, depending on the height of the interface, between 1and 6. Other angles are also possible and give rise to other values ofQ. In FIG. 21 these values and intermediate values are plotted along theslanting side 82. At the topside number-values for Q have been denotedfor the maximum that can be reached when the angle is modified and thusthe inclination of the slanting line of the triangle. If this slantingline is given an angle α₁ and α₂ respectively, the value of the maximumQ is 2 and 9 respectively.

The electrode 85, shown in FIG. 21A, has a slanting side 86 with astepwise profile. This electrode 85 can be thought to be composed of anumber of narrow sub-electrodes 87,88 etc. of ever-decreasing length orin the reverse case: of ever-increasing length, which are combined toone electrode-output. The electrodes 84, 85 are placed into a measuringtube, measured with respect to said measuring tube and connectedtherewith unbreakably. The measuring tube and electrodes as a whole areplace into a vessel.

In FIG. 22A is shown a level gauge according to the invention in sideview, also in plan view (FIG. 22B) and in bottom view (FIG. 22C).Contrary to the previous embodiment--but also destined for a normaltwo-liquid system--where the surface of the measuring electrode, whenconsidered in an upward direction, increases faster than that of thecompensation electrode, and where further distance between theelectrodes and the associating wall of the measuring tube placed aroundsaid electrodes, remains constant, is here the distance betweenmeasuring electrodes 101 and associating wall parts 106 of the measuringtube 110 increasing, whereas the ratio of the surfaces of the measuringelectrodes 101 and the compensation electrode 103 remains constant.

The measuring electrode 101 forms with the slanting wall part 106 aconductive cell G_(M) with variable distance 1. The compensationelectrode 103 forms with the wall part 108 a conductive cell G_(R) withconstant carrier 1. The electrodes 101, 103 are applied on a carrier105. The measuring tube 110 is grounded at 111. On the upper side of themeasuring tube 110 a measuring circuitry 115 is present, including twooscillator circuits 112, 113 and a divider circuit 114. The oscillatorcircuits 112 and 113 resp. are connected to the conductive cells G_(M)and G_(R) resp. and furnish a signal p and q resp. to thedivider-circuit 114. The conductance G_(R) is ##EQU11## that of G_(M) is##EQU12## wherein A=h_(i).r (for r, see FIG. 22B)=electrode surface, andχ=specific conductance of the liquid to be measured. For the distance lshould be taken the average value of the distance l_(o) in the lowestpoint of the electrode and the distance l_(i) and the highest point ofthe liquid at a given moment.

Since the variable l occurs in the denominator of the conductanceformula, one has to determine the quotient Q'=q/p, so that s_(avg)=s=(s_(o) +s_(i)) appears in the numerator of the quotinet. This meansthat an increasing liquid height h_(i) is accompanied with an increasingvalue of s. It follows that the height to be determined is directlyproportional with the measured quotient Q, which is a linear function ofthe distance.

FIG. 23A-C illustrates an alternative embodiment of the invention inFIG. 1, likewise with a side view (FIG. 23A(), plan view (FIG. 23B) andbottom view (FIG. 23C) and with an electric measuring circuit 115. Asfar as there is question of equal parts as in FIG. 22A-C, the samereference numerals will be applied. Parts with a differentconstructionform, have received another reference numeral. That is thecase with the measuring electrode, now 121, which is not straight, butinclined. Further the measuring tube 130, which instead of the slantingside 106, has a straight side 126. The carrier 125, which was incross-section rectangular, is now in cross-section a trapezium. Alsohere the distance between measuring electrode and cooperating side ofthe measuring tube increases with rising liquid.

In FIG. 24A-C is shown again an alternative form of the level gauge ofFIG. 22 in side view (FIG. 24A), plan view (FIG. 24B) and bottom view(FIG. 24C), in which equal part have been indicated with the samereference numerals. In this case the wall 106' of the measuring tube110' cooperating with the measuring electrode 1 is tapering in an upwarddirection, so just the reversal of FIG. 22A-C and 23A-C.

This situation is the reverse of that of FIG. 22A-C and 23A-C, whenwatching the distance l_(i). If one wishes to apply this arrangement ina normal liquid system, then the quotient Q=P/q must be determined toensure that a linear relation is measured between the height h_(i) andthe distance l_(i). This case is shown in the measuring circuit 115.

Via a small modification it is possible to determine by means of anothersub-circuit 116, to which are supplied the signal q and the result ofthe division Q carried out in the circuit 114, the constant χ of theliquid, so that the presence, if any, of traces of water in the oil,which is present between the electrodes of the capacitor, can bedetected.

If, on the contrary, the signals p and q are supplied to thedivider-circuit 114 such that the quotient Q=q/p is measured, then asimple calculation teaches that this kind of construction is suitablefor a "reversed" liquid system, which will be explained with referenceto FIG. 25A-F.

From all this, it follows that a given arrangement being suitable for anormal liquid system, need not be reversed per se in order to besuitable for a "reversed" liquid system. It suffices to exchange theconnector pins for the signals p and q mutually in the divider-circuit114.

FIG. 25A and B represent the level gauge for use in a "normal"two-liquid layer system. In FIG. 25A is supposed that the vessel is fullof oil, and in FIG. 25B full of water. At the place of the interface 132in FIG. 25A, the quotient of G_(R) and G_(M) is: ##EQU13## At the placeof the interface 133 in FIG. 25B the quotient Q is: ##EQU14## So arising water level is detectable by an increasing value of N.

FIG. 25C and 25D show schematically the case that the level gaugeaccording to the invention is applied in a "reversed" two-liquid layersystem, in which the usual ECH is heavier than water and is thus belowthe interface. In FIG. 25C it is supposed that the measuring vessel isalmost completely filled with ECH, so that the interface 134 is in theupper part of the vessel. In FIG. 25D, on the contrary, the vessel isnearly completely filled with water and so the interface 135 isvirtually in the lower part of the vessel. At the place of the interface134 the quotient Q=q/p has a high value, for example 10, whereas at theplace of the interface 135 the quotient Q has a low value, for example1.

Thus with rising interface, the quotient Q decreases. That is illogical.Therefore in FIG. 4E and F the measuring tube has been reversed.

Now in FIG. 25E a small quantity of H₂ O, so when the interface 136 lieshigh in the vessel, is accompanied with a low value of Q, for example 1.In FIG. 25F, when the vessel is almost completely filled with water, theinterface 137 lies down in the vessel and that means a high value of Q,for example 10. Hence an increasing quantity of water is now accompaniedwith a greater value of Q.

FIG. 26A-E show a second embodiment of the invention in a front view(A), side view (B), plan- and bottom view (C, D). The measuringelectrode A and 141 and the compensation electrode B or 143 areconstructed as a double, that means: they are provided on both sides ofa four sided electrode carrier 145. Due to this symmetrical constructionof the electrodes on the carrier 145, not only the precision is improvedin first instance, but also the response-sensitivity; this can lead to astill higher precision. The electrodes of like function are oppositeeach other and are connected-through electrically, as clearly shown inFIG. 26C. Around the whole assembly a measuring tube 146 has been place,which is straight, because the measuring electrodes 141 are provided onthe slanting sides of the carrier 145.

In FIG. 27A-D the electrode carrier 145' is reversed with respect to theelectrode carrier 145 of FIG. 29 and can therefore be used in a"reversed" liquid system.

FIG. 28A-B show a variant of the electrode carrier 145 of FIG. 26, whichis there in cross-section rectangular, whereas this variant is here incross-section triangular. In FIG. 28A a plan view is shown and in FIG.28B a perspective view. In this variant an electrode carrier 150 isshown with three sides 147,148 and 149. The sides 147 and 148 areinclined and carry the measuring electrodes M and M'. The third side isstraight and carries the compensation electrode R.

Not only in FIG. 28B but also in the plan view of FIG. 28A, it isclearly shown that the cross-section triangle 152 at the bottom has agreater surface than the cross-section triangle 153 at the top. In FIG.28A is also drawn with a dotted line, around the triangular electrodecarrier 150, the measuring tube 154, having the shape of a rectangular,three-sided parallellepiped.

It will be clear that also a conically shaped or other body of therevolution is very suitable for use in a level gauge according to theinvention. In FIG. 28C such a conical level gauge is represented atleast as far as the measuring tube is concerned. The electrode carrier155 is, however, cylindrical. At the place where the compensationelectrode 157 is provided, also the conical measuring tube has astraight generatrix 158. Also here again the electrode carrier can bemade conical, for example such as the body 154, and provides thereabouta straight measuring tube, such as indicated with dotted lines 159.

FIG. 29A-E show various possibilities to combine two different levelgauges, so that they are suitable for interface measurements in acombination of a "normal" and a "reversed" two-liquid layer system.

In FIG. 29A is shown the combination of a level gauge 161 for a normalliquid system and a level gauge 162 for a reversed liquid system. Sincethe measuring electrode 163 is straight, the cooperating sides 166, 167of the measuring tube sections 168, 169 ae inclined and the measuringtube--as-a-whole--exhibits a convex (or salient) kink.

FIG. 29B shows a combination of a level gauge 171 for a normal liquidsystem and therebelow a level gauge 172 for a reversed liquid system.Thereby the measuring tube--as-a-whole--175 exhibits a concave (orre-entrant) kink.

In FIG. 29C and D the kink in present in the combined measuringelectrode 177 and 178 resp.

In FIG. 29E is shown that also in such a combination the compensationelectrode can have a kink, provided the opposite wall 182 of themeasuring tube 185 exhibit the same inclination and kink. Of course, thewall 183 opposite the measuring electrode 186 should also have a kink.

If such a level gauge must bridge-over great differences in height foronly one liquid system, the dimensions in the transverse directions (dueto the inclination of the slanting sides) can become unacceptably great.It is recommended to off-set the exterior plate of the conductive cellfor example every few meters. This has been done in FIG. 30A, B for thetotal measuring tube 190 (thus the "external" conductive cell plate). Itconsist here in fact of 107 individual measuring tube sections 191, 192etc. of for instance each 1, 5 m of length, in which the measurementelectrode 95 itself is correspondingly divided into sub-electrodes 196,197 etc. being mutually insulated. The measuring electrode 195 hasbecome, by this fact, an electrode with sub-electrodes, in which eachsub-electrode takes care of a part of the scale-range. After passingcompletely a sub-electrode of the electrode 195, the gauge indicates100%; thereafter the next sub-electrode begins again at 0% and increasesto 100%, etc. The sub-electrodes are then summed numerically.

EXAMPLE

Coverage with liquid by 4 sub-electrodes each of 250 mm length. Thefifth sub-electrode is covered for 50%. The indication is then: 4×250mm=1000 mm +1/2 sub-electrode 5 (=125 mm); eventual height is then1000+125=1125 mm.

I claim:
 1. In a vessel holding a liquid, a gauge for measuring theconductance of a liquid in the vessel, comprising a measuring electrodewithin and spaced from a corresponding part of the vessel, acting as acounter-electrode, and a compensation electrode within the vessel whichis spaced from the measuring electrode and from a corresponding part ofthe vessel, acting as another counter-electrode, the electrodes andtheir counter-electrodes constituting a measuring and a compensationconductance cell therebetween, characterized in that the electrodes ofthe compensation cell are parallel, the measuring electrode is spacedfrom its opposite counter-electrode and the horizontal distance betweenmeasuring and counter-electrodes varies with the height at which suchdistance is measured, while the horizontal distance between compensationand counter-electrodes is constant, whereby the quotient of thecell-conductances varies as a linear function of the liquid height inthe vessel.